Large amounts of wave energy are potentially available, with especially favorable locations in, for example, the UK, Ireland, the USA, Australia and New Zealand. Wave energy can be produced with low carbon emissions and can be part of the solution to the risks of global warming.
There has been much interest in wave energy. Around 1,500 wave energy patents have been published worldwide over the last 100 years. Around 50 companies worldwide are trying to develop wave power. So far there has been no successful commercial megawatt-scale generation of power from waves.
The main obstacles to such implementation are poor reliability, high cost, low efficiency and environmental impact.
Reliability and cost problems arise from the oceanic environment, which is corrosive, erosive, laden with encrusting life-forms and which, in extreme weather, is capable of delivering massive mechanical stress.
Costs are high where the design of the wave energy converter (WEC) relies on over-engineering to counter extreme conditions. It is better to have a compliant design that mimics nature's solutions to ocean conditions or to minimize vulnerable elements of the WEC design or to use an adaptive approach where, for example, in extreme weather, the WEC submerges.
Costs are also high if extensive underwater work is required: for example to fix a rigid structure to the seabed. If a structure already exists, for example to support a wind turbine, this can be exploited. Structural costs can be reduced by using slack tethering to anchors on the seabed.
Low efficiency arises especially from non-resonant coupling between the WEC and the wave. In the worst case, the WEC and the wave are out of phase and energy is subtracted rather than added so that the instant efficiency is negative. Out of phase behavior is guaranteed where the waves are stochastic. Such waves occur in regions of sea that are (a) sheltered from the major oceans so that waves are mainly driven by local winds (b) in shallow waters where sea-bed reflection and refraction confuses the wave pattern (c) close to shore where reflection, diffraction and refraction also confuse the wave pattern. The distorting effects of seabed and shoreline are increased by irregular geometry. Distortion of wave motion by the seabed is almost zero where the sea depth is greater than half the wavelength. The typical peak of the annual ocean swell energy spectrum is 8-10 seconds period. This corresponds to a half wavelength of around 50-80 meters. Therefore an efficient WEC will be located in ocean that is at least 40 m deep and usually at least 1 km offshore.
An efficient WEC must be located in a region where there are strong persistent swells: in brief, offshore from a good surfing beach. Useful swells arise from prolonged central Atlantic and Pacific ocean storms of the kind generated by the Coriolis effect midway between the equator and the poles. Storm waves generated in mid-ocean decompose into persistent swell trains each of uniform period. Since long-period swells travel faster than short-period swells, the swell trains separate. Such trains are capable of traveling thousands of miles with minor energy loss. By using satellite synthetic aperture radar, swells can be tracked and arrival can be predicted many days ahead.
An efficient WEC will also be oriented to the dominant swell. This can be done in three ways: (a) by fixing the WEC in a zone where the dominant swell has a persistent direction (b) by using an omnidirectional device (c) making the WEC self-aligning.
Exposure to persistent energetic swells is a necessary but insufficient condition for an efficient WEC. It is also necessary for the WEC to be dynamically tunable over a wide range, where wide range means the range of periods that characterizes energetic ocean swell: around 5 to 15 seconds.
Dynamic tuning means tuning during operation and tuning rapidly between different swell periods. The required speed of tuning depends on the duration of swell trains, the variability of swell period, the predictability of swell period, the cost of tuning delay and the cost of tuning. The duration of swell trains depends on the duration of the storms that cause them: for example, around 70% of Atlantic tropical storms last more than two days (an average over the last 50 years).
Shifts of swell period tend to be in steps of only one to two seconds. Preferably it should be possible to shift the period of the WEC by one second within two minutes. Depending on all the factors mentioned, the preferable one-second tuning time for a specific configuration of WEC in a specific location may be between 10 seconds and 10 minutes.
Dynamic tuning is useful not only to achieve high efficiency but also to protect the WEC. When the amplitude of oscillation becomes excessive in extreme seas, the WEC can be quickly detuned.
Low efficiency can also result from single-vector energy capture. The water particles of a wave move orbitally, carrying energy in two vectors: vertical and horizontal. A heaving (or bobbing) WEC can capture only the vertical vector; a surging WEC can capture only the horizontal vector. Such devices have a maximum efficiency of 50%. Two-vector energy capture ie pitching offers a wave-to-WEC energy transfer that is up to 100% efficient. Despite the apparent advantage of the pitching vector, heaving and surging WECs represent the majority of WECs offered (in 2012) by wave energy companies. This bias may reflect a belief that heaving and surging WECs offer lower life-cycle costs.
For efficient capture of wave energy the WEC should engage with the swell at or near the ocean surface. At periods that characterize energetic swell, the available swell energy falls by around 2% for every meter of depth. Preferably the WEC should engage the swell within the top 10 meters.
Lastly, a successful WEC must be environmentally acceptable. This tends to limit the application of designs that are either on-shore or near-shore since there are few such locations near to any population centers where a large-scale installation would be tolerated.
Difficulty of Wide Range Dynamic Tuning
Although large WEC efficiency improvements are possible using resonant energy transfer, there has been little progress in achieving this. The equation for the period of an oscillating floating body helps to explain why:TB=2π√((IM+IA)/IC)  1.Where:TB is the period of the body in seconds.IM is the mass moment of inertia (MOI) in the direction of oscillation.IA is the added mass MOI of the body in the direction of oscillation.IC is the moment of the coefficient of restoring force.
Note:                Added mass is the inertia of fluid displaced by a body that is accelerating or decelerating. For example, a flat-bottomed vessel in pitching motion displaces a relatively large volume of water ie the added mass is large. For the same enclosed volume, a V-shaped body in motion displaces a smaller volume of water ie the added mass is small.        The restoring force is the net buoyancy. The coefficient is the rate of restoring force. It is proportional to the water-plane area        
For TB to vary by 3×, the bracketed function on the right of equation 1 must vary by 9×. At first sight, the solution is obvious: for any given value of IM and IA, simply vary IC. For example, if we steadily submerge a heaving vessel with a conical top, then the water-plane area will steadily diminish and TB will rise. However, the power from the WEC varies with restoring force, stroke length and period. By reducing IC, the power capacity is also reduced. In specific conditions that are included in the present invention, this problem can be overcome. But generally, we want IC to be fixed and we must vary IM+IA.
For IM+IA to vary by 9×, either:                One of these terms is very small and the other term can vary by 9×, or        Both terms can vary by 9×        
It is easy to show that if either term is fixed and not small, or if either term has a fixed component that is not small, then the required variation in the other term can be much larger than 9×.
If we consider a heaving body, then equation 1 simplifies to:TB=2π√((M+MA)/C), where:  2.M is the massMA is the added massC=A×d×g where A is the water-plane area, d is the density of seawater and g is the gravitational constant.
If, say, the body is a simple upright cylinder, then TB cannot be varied by 3×, due to the significant fixed value of MA. If MA is reduced by streamlining, we may need a 10× variation in M.
An obvious method of varying M is to pump seawater into the cylinder. The amount required for a 10× variation in TB depends on the value of M when TB=5 seconds.
For example, if the cylinder has a diameter of 5 m, then A=19.6 m2 and from equation 2, approximately:2π√((M)/19.6 g)=5M=123 tonnes, equivalent to a draft of around 6 m
To raise TB to 15 seconds requires increasing this mass by 10×: on average pumping 123 tonnes of water for every second of change in TB. This is operationally costly and slow.
The present invention shows how to achieve wide-range dynamic tuning without (a) sacrificing power output and (b) at low operational cost and high speed.